Finer grained migration control

Finer grained migration control

Normally, when you migrate a domain from host A to host B,
the domain on host A remains defined but shutoff and the domain
on host B remains running but is a "transient".  Add a new
flag to virDomainMigrate() to allow the original domain to be
undefined on source host A, and a new flag to virDomainMigrate() to
allow the new domain to be persisted on the destination host B.

Signed-off-by: Chris Lalancette <clalance@redhat.com>